The recovery of zinc from zinc sulphate solution usually takes place electrolytically. Chloride is a fairly harmful substance in zinc electrolysis, since it corrodes the cathode surfaces so that problems are caused when peeling the zinc plate away from the cathode.
U.S. Pat. No. 4,005,174 is known in the prior art, which describes the removal of chloride from a solution that has been obtained by leaching the slag generated in galvanisation into a solution of sulphuric acid. The amount of sulphuric acid in the solution is adjusted so that the pH of the solution is always below 2.6. Chloride removal is performed using copper(I) oxide (Cu2O). Three ways of forming copper(I) oxide in zinc sulphate solution are presented in the publication and all removal methods are presented by means of one process flowsheet. According to the first alternative, copper sulphate and zinc powder are added to the chloride removal stage, reacting to form cuprous oxide, which then reacts further with chloride to form copper chloride, which is precipitated from the solution. According to the second method, copper sulphate and metallic copper powder are added to the solution, and as a result of the reaction between them, monovalent copper is also generated. According to the third alternative, copper(I) oxide is added to the solution, which reacts with chloride. All these alternatives carry the condition that after chloride removal at least 0.5 g/l of copper sulphate should remain in the solution. The chloride is washed from the copper chloride residue by means of sodium hydroxide, the sodium chloride solution that is formed is removed and the copper(I) oxide generated is fed back to chloride removal. The excess copper in the zinc sulphate solution that is present after chloride removal is removed by cementing it with zinc powder, and the metallic copper obtained is oxidised by means of sulphuric acid and air into copper sulphate, which is fed back to the chloride removal stage.
It has been found in practice, that feeding copper(I) oxide directly into the chloride removal stage is the fastest way to remove chloride from a solution, because when feeding copper sulphate and zinc powder into a zinc sulphate solution, generally the reaction in which copper is reduced to metallic copper occurs first and only then is it oxidised into oxide. The generation of copper(I) oxide in zinc sulphate solution in the method accordant with U.S. Pat. No. 4,005,174 is probably fairly slow, because the pH of the solution is specified to be below 2.6, which is too low for effective formation of copper(I) oxide. It has also been found that in addition to copper(I) oxide being formed in the chloride washing subsequent to chloride removal, metallic copper is also formed, which remains circulating in the process unless it is leached and removed.
A method according to FI patent publication 117 246 is also known in the prior art, which describes both chloride removal by means of copper(I) oxide and copper removal by ion exchange from concentrated zinc sulphate solution. The method is particularly suitable for raw solutions that have a copper concentration high enough for the copper contained in the solution can be used for the fabrication of copper(I) oxide used to remove chloride from the solution. The use of the copper in the raw solution is optimised further by recycling.